Machine tools that improve positioning accuracy by driving moving bodies such as a table and bed in a non-contact manner using a linear motor at the same time as guiding the moving body in a non-contact manner using hydrostatic air bearings, are known. These types of machine tools are used, for example, in order to manufacture dies for components of optical equipment requiring extreme degrees of miniaturization. They are also used in the fine manufacture of diffraction gratings for semiconductor lasers on the surface of a substrate formed from high hardness materials using a diamond tool. Mechanical methods using machine tools have advantages over chemical methods such as photoetching, namely that it is possible to repeatedly manufacture a diffraction grating having excellent durability. However, in order to mechanically manufacture a diffraction grating having groove spacing of less than 1 μm, there is a need for a higher precision machine tool provided with a positioning device capable of positioning in sub-micrometer or nanometer units.
A non-contact type position detection element is desired in a high precision machine tool to measure displacement amount of a moving body without imparting resistance to the movement of the moving body. Such a type of position detection element can utilize the advantages of a machine tool having a combination of hydrostatic air bearings and linear motor, namely infinitesimal frictional resistance. Non-contact type position detection elements mainly used in machine tools are, for example, optical linear encoders and magnetic linear encoders. These linear encoders comprise an elongated sheet scale having engraved gradations of the same pitch, and a read head for optically or mechanically reading gradations on the scale. One of either the head or the scale is attached to the moving body, and the other is attached to a support body. The support body is, for example, a bed and column for supporting the moving body using hydrostatic air bearings.
A higher precision machine tool is provided with a linear encoder having lattice gradations on a finer pitch. If the resolution of the linear encoder is made higher, even a small amount of microscopic dust and chips etc., attaching to the scale will have a more than negligible effect on the high position accuracy. There is a possibility that attachment of dust may damage the scale surface. This causes errors in detection due to variation in the amount of transmitted or reflected light detected by a read head of an optical linear encoder.
Japanese Patent Laid-open No. 11-063966 discloses a magnetic linear encoder having a scale and a read head arranged inside a long metal case. The case has a cross section that is U-shaped, open at the bottom, and dust is prevented from getting into the inside of the case by blocking off the open surface of the case with a rubber dust lip.